While a number of effective chemotherapies are available, tumors inevitably evolve resistance to these drugs ultimately resulting in treatment failure and cancer progression. Causes for chemotherapy failure in cancer treatment include poor blood supply to the tumor, low tumor oxygen concentrations, and resistance to drug-induced toxicity through up-regulated metabolic pathways or DNA repair mechanisms.

Unless a cytotoxic therapy eradicates all cancer cells, its application to a tumor population also produces evolutionary selection forces that will select for cells that are